At its present stage of development, the precision machining field is in a state of flux. Systems that are totally dependent on manual operations have largely given way to techniques whereby manufactured parts are made with general purpose, numerically controlled machining systems. Although cutting or other removal of material occurs automatically in such systems, numerous manual operations are still required, primarily for measuring the machined dimensions, and for making cutter adjustments using an ordinary numerical control cutter offset. These manual measurements and adjustments of the cutting tool are necessary to take into account a large number of variables, such as wear of the cutting tool, repositioning and/or replacement of the cutting tool, as well as dimensional changes of the cutting tool, the work piece and the machining apparatus itself due to such factors as heating, deflection under load and the like.
By way of example, in a typical operation carried out with a numerically controlled machine tool such as a lathe, certain adjustments, i.e., tool offsets, must be manually implemented by the operator after the machine is set up for the manufacture of a particular work piece or part. Prior to the start of machining the operator must advance the cutting tool to a tool setting surface and determine the tool position by manually measuring the space between the tool and the reference surface. This is normally done with a piece of shim material, or the like and such measurements then form the basis for manually making tool offsets. Where the lathe includes tool holding means such as a multiple tool turret, this operation must be carried out separately for each tool, as well as for each of the axes (of motion) of the machine. Prior to making the final or finishing cut for a particular work piece surface, the various dimensions of the semi-finished work piece surface are measured by using a hand-held gauge. This enables the operator to determine the required offset of the cutting tool which is used for the finishing cut. After the finishing cut is made, the work piece is again checked with the hand-held gauge in order to measure the conformance of the actual dimensions of the finished surface to the desired dimensions.
The manual operations described above are individually time-consuming and take up a significant amount of the total time required to machine a particular work piece to the desired dimensions. This serves to limit the manufacturing capacity of the machine tool. Considering present day costs of a lathe or a milling machine (machining center), any reduction of the capacity of the machine tool becomes a matter of economic significance. Further, all such manual operations are prone to introduce errors into the manufacturing process.
As is generally recognized, the solution to the foregoing problems is to automate manual measurements and the manual adjustments of the cutting tool, e.g., by the use of a computer-operated numerical control system. In such a system the computer may either be positioned remote from the numerical control unit, or it may be incorporated in the latter, e.g., in the form of a microcomputer. Alternatively, a computing capability may be provided remote from the numerical control unit as well as being incorporated into the latter. Instead of down-loading successive blocks of data stored on tape or the like as is the case in an ordinary NC system, a computer numerical control (CNC) system is capable of storing entire programs and calling them up in a desired sequence, editing the programs, e.g., by addition or deletion of blocks, and carrying out the computations of offsets and the like.
Although fully automatic systems have not been widely adopted at this stage of development of the precision machining field, a considerable amount of development work has been done to date, much of it limited to special purpose situations wherein a single machining operation is repetitively carried out. It is also known to mount a contact probe or tool sensor on the bed of the machining apparatus, or on a pivotal arm that can be swung out of the way when desired. The position of the cutting tool can be calibrated against such a probe by noting the tool position when contact with the probe occurs. From the observed deviations between the programmed and the actual positions, a compensating offset may be determined and stored in the memory associated with the computer numerical control means. The offset compensates for the difference between the programmed contact position and the actual contact position. Further, it is known to mount a contact probe or part sensor in the tool holding means; to calibrate such a probe against a reference surface on the machine; to probe the machined surface of the work piece and to derive from such probing the information for determining the final offset required for the finishing cut; and to probe the finished surface for conformance with the desired dimensions.
Although some of the foregoing operations have been automated in the past by means of closed loop numerically controlled machining systems, many of these systems fail to provide the necessary accuracy for high-precision machining operations, for example, for machining high-precision parts for aircraft engines or the like. Such improvements as have been implemented to date to enhance the precision of closed loop numerically controlled machining systems often require special purpose equipment which may materially increase the total cost of the system. To the extent that the complexity of the overall system is increased by the use of such equipment, system reliability may be diminished with attendant adverse effects.
An example of a system in which complex special purpose equipment is used in order to enhance the obtainable precision of the machining operation is given by U.S. Pat. No. 4,195,250. The patent discloses an arrangement which uses two probes or styluses operating under numerical control. These styluses are alternately brought into contact with a pair of points selected to lie opposite each other, e.g., diametrically opposite each other on the work piece which is to be measured. The deviation of the desired value from the actual value is determined and is used as tool position compensating data. As stated in the patent, such a measurement can be achieved independently of such factors as main spindle displacement caused by the thermal displacement of the machine tool, or by the non-coincidence of the machine coordinates with the program coordinates. Clearly, the techniques described in the patent requires apparatus whose complexity tends to raise the cost of the overall system. Due in part to the complexity of the equipment and technique employed, errors may occur as explained in the patent. For example, the heat generated by the motors which move the cross slide is likely to affect the positioning accuracy.
Another arrangement disclosed in the same patent shows a single stylus which moves under numerical control and which is alternately brought into contact with the aforesaid opposite points on the work piece. The amount of movement of the stylus head between the two points is measured for the period of time required and this measurement is added to those already in the numerical control means to determine the actual dimension between the points, as well as the required compensation of the tool position. Here again, special purpose equipment is required with the attendant disadvantages discussed above.